Christian churches are strongly against the use of violence, and as in the Just War criteria, war is seen as the last resort. Some churches are actually against war in all circumstances. So, the Church teaching on violence mirrors the teaching on war.
There is a school of thought within Christianity, known as Liberation Theology, that holds that Christians should bring about justice for the poor through political activism.
Roman Catholic Church
Many Catholics have taken an active role in issues of social justice, medical ethics, the environment etc. For example, environmental activist and 'martyr' Chico Mendes was recruited by the Catholic-supported Confederation of Agricultural Workers in the 1970s. Have a look at the collection of Roman Catholic Activists from Wikipedia.
The roots of Liberation Theology are largely Catholic. This picture from the 1930s, by Jose Clemente Orozco, shows how Jesus was seen by many people in developing countries as a Liberator.
Church of England
The Church of England has repeatedly taken a stand on national and international issues. For an example of Anglican activism, have a look at this pdf file on Ethical Consumerism. There is also a document on the Church's Ethical Investment Policy. There are also Policy Statements on a range of issues.
